The present application relates to magnetoresistive random access memory (MRAM). More particularly, the present application relates to a magnetic tunnel junction (MTJ) structure including a multilayered magnetic free layer structure that can improve the performance of spin-transfer torque (STT) MRAM.
MRAM is a non-volatile random access memory technology in which data is stored by magnetic storage elements. These elements are typically formed from two ferromagnetic plates, each of which can hold a magnetization, separated by a thin dielectric layer (i.e., a tunnel barrier). One of the two plates (i.e., the magnetic reference or pinned layer) is a permanent magnetic set to a particular polarity; the other plate's (i.e., the magnetic free layer's) magnetization can be changed to match that of an external field to store memory. Such elements may be referred to as a magnetic tunnel junction (MTJ) structure.
One type of MRAM that can use such a MTJ structure is STT MRAM. STT MRAM has the advantages of lower power consumption and better scalability over conventional magnetoresistive random access memory which uses magnetic fields to flip the active elements. In STT MRAM, spin-transfer torque is used to flip (switch) the orientation of the magnetic free layer. Moreover, spin-transfer torque technology has the potential to make possible MRAM devices combining low current requirements and reduced cost; however, the amount of current needed to reorient (i.e., switch) the magnetization is at present too high for most commercial applications.
In the prior art of spin torque switching, the emphasis has been on lowering the magnetic damping (also called Gilbert damping) of the magnetic free layer. The theory suggests that the switching current is directly proportional to the damping; see, for example, J. Z. Sun, Phys. Rev. B 62, 570 (2000). Hence lower damping makes the free layer switch in lower current, which is desirable since it means a smaller cell transistor can be used.
FIG. 1 illustrates a prior art MTJ structure that has been developed in order to reduce the current needed to reorient (i.e., switch) the magnetization of the active elements. The prior art MTJ structure includes a multilayered magnetic free layer structure 15 that contains two magnetic free layers (14 and 18) separated by a non-magnetic layer 16 as is shown in FIG. 1. FIG. 1 also includes a magnetic reference (or pinned) layer 10, and a tunnel barrier layer 12. Element 14 is the first magnetic free layer that forms an interface with the tunnel barrier layer 12, while element 18 is the second magnetic free layer that is separated from the first magnetic free layer 14 by the non-magnetic layer 16. In the drawing, the arrow within the magnetic reference layer 10 shows a possible orientation of that layer and the doubled headed arrows in the first and second magnetic free layers (14 and 18) illustrate that the orientation in those layers can be switched. The non-magnetic layer 16 is thin enough that the two magnetic free layers (14 and 18) are coupled together magnetically, so that in equilibrium the first and second magnetic free layers 14 and 18 are always parallel.
One drawback of the prior art MTJ structure shown in FIG. 1 is that the switching of the multilayered magnetic free layer structure 15 can be too slow in comparison to the length of the applied voltage pulse. This ‘drag’ in switching of the orientation of the multilayered magnetic free layer structure 15 of the prior art MTJ structure of FIG. 1 may result in a write error.
There is thus a need for providing MTJ structures for use in STT MRAM technology which include an improved multilayered magnetic free layer structure that substantially reduces the switching current needed to reorient the magnetization of the multilayered magnetic free layer, while improving the switching speed and even reducing write errors of the STT MRAM.